1. Field
Exemplary embodiments of the present invention relate to a differential amplifier circuit, and more particularly, to a technology for improving asymmetry of output signals in a differential amplifier circuit (a buffer circuit) that receives pseudo-differential input signals.
2. Description of the Related Art
FIG. 1 is a circuit diagram illustrating a conventional differential amplifier circuit 100.
Referring to FIG. 1, the conventional differential amplifier circuit 100 includes a differential amplification unit 110 and a current source 120.
The differential amplification unit 110 amplifies a voltage difference between an input terminal A and an input bar terminal B and generates an output signal OUT and an output bar signal OUTB. The current source 120 controls a constant amount of current to flow through the differential amplification unit 110.
FIGS. 2A and 2B are diagrams illustrating waveforms of output signals OUT and OUTB according to waveforms of input signals input to the input terminals A and B of the differential amplifier circuit 100.
Referring to FIG. 2A, an input signal IN is input to the input terminal A and an input bar signal INB obtained by inverting the input signal IN is input to the input terminal B. Such an input is called fully-differential input. Since the fully-differential input signals IN and INB have symmetric waveforms, the output signals OUT and OUTB of the differential amplifier circuit 100 also have symmetric waveforms, similarly to the input signals IN and INB.
Referring to FIG. 2B, an input signal is input to the input terminal A and a reference voltage VREF is input to the input terminal B. Such an input is called pseudo-differential input. The pseudo-differential input signals IN and VREF have asymmetric waveforms because the reference voltage VREF is not oscillated, and the output signals OUT and OUTB of the differential amplifier circuit 100 also have asymmetric waveforms. Particularly a swing width of the output signal OUT is narrower than that of the output bar signal OUTB, which may results in the generation of signal distortion, such as amplitude (DC) distortion, time (AC) distortion, or a duty error, in a circuit using the output signal OUT and the output bar signal OUTB at a rear stage of the differential amplifier circuit 100.
In order to alleviate the concern illustrated in FIG. 2B, the fully-differential input is used. However, since some of integrated circuit chips may not receive differential input signals and may receive only single-ended input signals in many cases, the use of the pseudo-differential input is required.
The paper ‘Single-Ended Transceiver Design Techniques for 5.33 Gb/s Graphics Application’ (ISCCC 2009/SESSION 7/DRAM/7.5), which is published by Hamid Partovi and the like as a study for resolving concerns arising when pseudo-differential input signals are input to a differential amplifier circuit, has proposed the addition of a compensation capacitor between an input terminal A and a common node C of the differential amplifier circuit. However, since the use of the compensation capacitor shows an actual effect only when an input signal IN is changed to an ultrahigh frequency and a swing level of an output signal is not constantly maintained, it is does not resolve the above concerns completely.